The disposal and utilization of rubber waste from various fields of industry is a continually growing problem both in terms of environmental pollution and reuse since the amount of rubber waste is continually increasing worldwide.
At present, for example, many millions of metric tons of used tires arise worldwide each year, about 25% of which are commercial vehicle tires. In Europe alone, 3.2 million metric tons of used tires arise every year at present, with the proportion of commercial vehicle used tires being about 0.8 million. A large part of the used tires is passed to utilization in terms of energy in the cement industry and is therefore no longer available for further use as material, despite an increasing shortage of raw materials.
Up to now, there are only very limited possibilities for utilization as material of rubber waste, in particular used tire material, in the form of granulated material or powder in mixtures for new products such as new tires. The most important use as material is the use of the granulated rubber or rubber powder, also referred to as ground rubber, of various particle sizes produced from rubber waste in rubber mats or other rubber products which are not subjected to dynamic loads.
The granulated rubbers or rubber powders from used tires can be added in small amounts as aggregate to freshly produced tire mixtures in tire production. However, the amount which can be used is very greatly restricted since the granulated particles act as small foreign bodies in the fresh tire mixture. The reason for this behavior is, inter alia, that strength and elongation of the used tire granules differ significantly from those of the fresh mixture after vulcanization in the tire mixture. The granulated tire particles are then vulcanized for a second time in the vulcanization of the new product. The first vulcanization of the granulated tire material was carried out previously in the used tire, the starting material for the granulated tire material. The used tire particles which have now been vulcanized for a second time display a difference in the strength/elongation behavior at the interface to the fresh tire mixture, which can have an adverse effect on the dynamic performance. In many rubber products which are not subjected to dynamic loads, for example, rubber mats, stands for mobile traffic signs on building sites or oblique curbstone driveways, granulated tire material or granulated material from other rubber products can be added to the fresh mixtures since in this case there is no dynamic stress as, for example, in the case of a tire.
To nevertheless be able to utilize rubber waste such as granulated used tire material to gain high value in rubber products, these can be regenerated in a known manner. In a regeneration process, the vulcanized granulated tire material is plasticized, that is, it is converted from the elastic state into a plastic state, by breaking the sulfur bridges formed by vulcanization. The regenerate formed can thus be added again as raw material to a fresh rubber mixture and be bound into this by vulcanization.
Apart from the breaking of the sulfur bridges, rubber chains, that is, carbon-carbon bonds, are also broken to a not inconsiderable extent in the regeneration processes known from the prior art. Such degradation behavior is usually undesirable. The shortened (degraded) polymer chains of the regenerate cause disadvantages in terms of the physical properties, for example, rebound resiliencies, in the vulcanizates produced therefrom, which is, for example, reflected in an increased rolling resistance of pneumatic vehicle tires.
There are numerous known processes for regenerating or devulcanizing vulcanized granulated or ground rubber. All processes have the objective of regenerating, that is, breaking up, the crosslinking structure. Sulfur bridges are broken by the action of thermal and mechanical energy on roll mills or in mixers and also with the aid of various sulfur-bridge-breaking chemicals. This procedure is generally referred to as regeneration. As indicated above, it is often necessary to accept the disadvantage that the molecules of the vulcanized granulated or ground rubber are additionally depolymerized. The now plastic product having a defined viscosity formed in the abovementioned processes is commercially available as regenerate for a variety of additions to tire and industrial rubber mixtures and displays the negative properties of unregenerated granulated or ground rubber in dynamically loaded products to a significantly reduced extent.
Processes for regenerating sulfur-crosslinked rubber vulcanizates with the aid of various substances and in various apparatuses have been known for a long time, with steam regeneration, mechanical regeneration, thermal regeneration, regeneration by means of sound waves, regeneration by means of radiation and chemical regeneration being known.
Typical regenerating agents are sulfides with or without amine substituents, for example, tolyl disulfides and dixylyl disulfides, mercaptan derivatives such as 2-mercaptobenzothiazole and also hydroxide ions. Such agents accelerate the oxidative cleavage of sulfur bridges of vulcanized elastomers in the regeneration. Furthermore, regenerating oils having a swelling effect, for example, tall oil or resin oil, which swell the vulcanizate to be regenerated and thus increase its surface area so that oxidative regeneration is accelerated, are known.
These agents, which have different modes of action, are often used in combination, as described, for example, in Römpp Online Version 3.26: “Prolonged heating of rubber vulcanizates (used tires) to 150-250° C. in the presence of regenerating agents such as ditolyl or dixylyl disulfide for chain cleavage and tall oil or resin oil for swelling gives regenerates which can be vulcanized again”.
The comparatively high temperatures which are necessary to carry out the regeneration by means of such agents are disadvantageous. High temperatures usually increase the process costs and have a disadvantageous effect on the length of the polymer chains.
WO 2008/148706 A1 discloses processes for regenerating sulfur-crosslinked rubber vulcanizates, in which at least one dialkyl polysulfide is used as regenerating agent. As dialkyl polysulfides, it is possible to use, for example, branched dioctyl pentasulfide or dioctyl tetrasulfide. The regenerates produced therewith also display disadvantages in respect of the physical properties, in particular the buildup of heat in vulcanizates.